GUIDELINES FOR THE ALLEVIATION OF EXCESSIVE SURGE PRESSURES ON ESD SOCIETY OF INTERNATIONAL GAS TANKER AND TERMINAL OPERATORS The Society is a non-profit making organisation dedicated to the protection and promotion of the mutual interests of its Members in the safe operation of liquefied gas tankers and liquefied gas loading and receiving terminals. SIGTTO was formed in October 1979 and currently has in membership over 60 companies owning or operating over 70% of world LNG carrier and terminal capacity and over 45 % of LPG carrier and terminal capacity. SIGTTO is organised to represent its membership before, and consult with, the International Maritime Organization and other bodies. The Society of International Gas Tanker and Terminal Operators Ltd., London Liaison Office, Staple Hall, 87/90 Houndsditch, London EC3A 7AX Tel 01-621-1422 Telex 894525 - G Fax 01-626-5913 First published in July 1987 by Witherby Seamanship International Ltd., 32-36 Aylesbury Street, London EC1R 0ET Copyright of S1GTTO, Bermuda July 1987 ISBN 0 948691 40 9 NOTICE OF TERMS OF USE While the recommendations and advice given in this guide have been based on information made available to SIGTTO, no responsibility is accepted by the Society of International Gas Tanker and Terminal Operators Ltd or by any person, firm, corporation or organisation who or which has been in any way concerned with the furnishing of information or data, the compilation, publication or authorised translation, supply or sale of this guide for the accuracy of any information or advice given herein or any for any omissions herefrom or for any consequences whatsoever resulting directly or indirectly from compliance with or adoption of any of the recommendations or guidance contained herein even if caused by a failure to exercise reasonable care. Printed in England by Witherby Seamanship International Ltd., London, EC1 CONTENTS Page No. Purpose of Guidelines 1 Summary 1 Definitions 2 PART A - SOME DESIGN CONSIDERATIONS AND OPERATIONAL GUIDELINES 3 1. Potential Surge Pressure Hazards 3 2. Guidelines Common to Ship and Terminal 6 2.1 Guidelines prepared by an industry working group convened by the International Chamber of Shipping (ICS) 6 2.2 Safe transfer rates 6 2.3 Valve characteristics/closure times 6 2.4 Pipeline design pressure and pipeline anchors 8 2.5 Linked ship and terminal shut-down signal systems 9 2.6 Operational factors 9 3. Additional Guidelines for Consideration by the Terminal 11 4. Additional Guidelines for Consideration by the Ship 11 PART B - THE THEORY OF SURGE PRESSURE AND ITS ALLEVIATION 13 5. Surge Pressure Theory 13 5.1 The Joukowski equation 13 5.2 Line pack effects 13 5.3 Secondary surges 13 5.4 Surge assessments 14 5.5 The calculation of wavespeed 14 6. Surge Pressures in a Typical Loading System 15 6.1 Description of "basic" system studied 15 6.2 Effect of independent ship ESD 16 6.3 Alleviation equipment 18 7. Surge Pressures in a Typical Unloading System 18 8. Effects of Variation of Parameters 19 8.1 Flow rates 19 8.2 Valves 20 8.3 Line lengths 22 8.4 Pipe diameters 24 8.5 Fluid properties 25 8.6 Dead legs 26 8.7 Gas eliminators 26 8.8 Pumps 26 9. Design Approach to Alleviate Surge Pressures 27 9.1 Pump shut-down/diversion of flow 27 9.2 Surge pressure relief 28 10. References 34 LIST OF FIGURES Page No. 1. Typical development of surge pressure after a rapid valve closure 4 2. Improvement in safe loading rate if flow rate is controlled at the pump discharge and not at the end of the jetty or on the ship 10 3. Start of surge pressure on closure of ESD valve 12 4. Full surge pressure following ESD valve closure 12 5. LPG tanker loading system 15 6. The reduction of flow with time on valve closure 21 7. LPG tanker unloading system 31 LIST OF GRAPHS A. Butane loading - Flow reduction at jetty head on ESD 17 B. Butane/ammonia - Variation of pressure at jetty head on ESD 17 loading C. Butane loading - Variation of maximum pressure with pumped flow rate 19 D. Butane loading - Variation of maximum pressure with Total Valve Closure Time 23 E. Butane loading - Variation of maximum pressure with ESD valve size 23 F. Valve closure - Variation of Effective Valve Closure Time with number of ESD characteristics valves closing in parallel 24 G. Butane loading - Variation of maximum pressure with line length 25 H. Butane loading - Variation of maximum pressure with pump trip delay 29 J. Butane loading - Reduction in flow at jetty head with pump trip 29 K. Butane loading - Variation of pressure on ESD with pump trip 30 L. Butane unloading - Variation of maximum pressure with pump trip delay 30 M. Butane unloading - Variation of pressure at jetty head on ESD with no pump trip 31 N. Butane unloading - Variation of pressure at jetty head on ESD with pump trip 33 ACKNOWLEDGEMENTS The Society acknowledges the assistance given by Hydraulic Analysis Limited in drafting the text of Part B of these Guidelines and for the preparation of the Figures and Graphs. Acknowledgement is also due to Members of the ESD working group. 2.5 Linked ship and terminal shut-down signal systems 2.5.1 Recommendations for linked ESP systems As described in the final paragraph of Part A, section 1, and in more detail in Part B section 9.1.2 of this document, considerable reductions in surge pressure can be achieved by linking ship and terminal shut-down systems, provided that both the link and the associated ESD systems are of high integrity. Reference 10.1 describes such a link system and these Guidelines will serve to assure the integrity of the ESD systems. Part A, Section 4 includes further considerations for a ship's interface with a link system. 2.5.2 Pneumatic, electric, radio and fibre-optic signal systems Pneumatic signal systems contain inherent Signal Response Times dependent primarily on the length of signal pipework between the venting point and the location of the ESD pressure sensing device. Such systems are acceptable for the provision of a robust simple means of initiating an ESD confined to the ship. Intrinsically safe electrical signal systems are delay-free and have been proven reliable in dedicated liquefied gas shipping trades and in marine oil/gas installations provided that they are adequately specified and operated (see the Appendix to Reference 10.1). Radio transmission links are delay-free and in use where both the ship and the terminal are equipped with a radio transceiver interfaced to their respective ESD systems. Such links have to be unaffected by any other electro- magnetic sources which may be present in the ship/shore environment, may not interfere with any communication systems of ship and terminal and use a locally approved transmission frequency. Fibre-optics is a relatively new signal transmission technology which is also delay-free. 2.6 Operational factors The following factors are well documented and understood but they are basic to the maintenance and control of a high integrity and safe emergency shut-down system and are repeated here to complete the picture. Good operating procedures should avoid pressure surge problems. Surge pressures are most likely to be created during cargo transfer as a result of one of the following occurrences: (a) closure of an automatic ESD valve. (b) rapid closure or opening of a manual or power-operated valve. (c) slamming shut of a non-return valve. (d) starting or stopping of a pump. The ESD system equipment should be regularly maintained and tested to ensure correct and reliable function. The inadvertent closure of a valve, without ESD initiation due to mal-operation or malfunction and without the protective features of pump shut-down, could result in hazardous surge pressures. This problem should be considered individually by ships and terminals in terms of pipeline design pressures, cargo transfer rates, operating procedures and what is reasonably practicable and safe in the circumstances. However, the availability of rapid shut- down of transfer pump pressure by reaction of the terminal or ship via a linked ESD signal system will be a major contribution to minimisation of cargo spillage or of potential equipment damage (see Part B, Section 9.1.2). - 9 - The ESD system should therefore be checked to ensure that all factors which can initiate ESD valve closure during a cargo transfer also actuate the ESD signal system. Manually-operated valves which can be closed against the cargo flow should be operated slowly (or not at all). Low gearing may be fitted to the hand operating wheels of such valves to prevent over-fast operation. Similarly, manually-operated valves which may be isolating a pressure difference should be opened slowly to limit the surge pressures consequent on rapid velocity changes in the line, especially when filling an empty line. Manual transfer pump start procedures, so as to ensure that empty lines are filled slowly, and stop procedures, after the transfer rate has been slowly reduced to zero, should be carefully followed. Transfer pumps are usually specified for their maximum duty rating for pressure head and volume flow. When operated at below duty back pressure, they will deliver at an increased flow rate. This will increase surge pressures on ESD. Transfer flow rate should be adjusted slowly by use of a flow control valve preferably at the pump discharge and not by closing a valve at the receiving tank. The ship should request the loading terminal to adjust the loading flow. The terminal should request the ship to adjust the discharge flow. Figure 2 from Reference 10.6 illustrates the increase in safe loading rate with flow controlled at the pump discharge if the ship shuts down against the loading flow. Figure 2 Improvement in safe loading rate if flow rate is controlled at the pump discharge and not at the end of the jetty or on the ship. (This Figure is from Reference 10.6, but with conditions adapted for butane). - 10 -